1. Field of the Invention
This invention relates generally to apparatus and processes in fluid catalytic cracking (FCC). More specifically, this invention relates to increasing the mixing in regenerators.
2. Description of the Prior Art
Fluid catalytic cracking, as well as Resid FCC(RFCC), is a catalytic conversion process for cracking heavy hydrocarbons into lighter hydrocarbons by bringing the heavy hydrocarbons into contact with a catalyst composed of finely divided particulate material. Most FCC units use zeolite-containing catalyst having high activity and selectivity.
The basic components of the FCC reactor section include a riser, a reactor vessel, a catalyst stripper, and a regenerator. In the riser, a feed distributor inputs the hydrocarbon feed which contacts the catalyst and is cracked into a product stream containing lighter hydrocarbons. Catalyst and hydrocarbon feed are transported upwardly in the riser by the expansion of the lift gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, upon contact with the hot catalyst. Steam or an inert gas may be used to accelerate catalyst in a first section of the riser prior to or during introduction of the feed. Coke accumulates on the catalyst particles as a result of the cracking reaction and the catalyst is then referred to as spent catalyst. The reactor vessel disengages spent catalyst from product vapors. The catalyst stripper removes absorbed hydrocarbon from the surface of the catalyst. The regenerator removes the coke from the catalyst and recycles the regenerated catalyst into the riser.
The spent catalyst particles are regenerated before catalytically cracking more hydrocarbons. Regeneration occurs by oxidation of the carbonaceous deposits to carbon oxides and water. The spent catalyst is introduced into a fluidized bed at the base of the regenerator, and oxygen-containing combustion air is passed upwardly through the bed. After regeneration, the regenerated catalyst is returned to the riser.
Oxides of nitrogen (NOX) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Regulated NOX emissions generally include nitric oxide (NO) and nitrogen dioxide (NO2), but the FCC process can also produce N2O. In an FCC regenerator, NOX is produced almost entirely by oxidation of nitrogen compounds originating in the FCC feedstock and accumulating in the coked catalyst. At FCC regenerator operating conditions, there is negligible NOX production associated with oxidation of N2 from the combustion air. Production of NOX is undesirable because it reacts with volatile organic chemicals and sunlight to form ozone.
The two most common types of FCC regenerators in use today are a combustor style regenerator and a bubbling bed regenerator. Bubbling bed and combustor style regenerators may utilize a CO combustion promoter comprising platinum for accelerating the combustion of coke and CO to CO2. The CO promoter decreases CO emissions but increases NOX emissions in the regenerator flue gas.
The combustor regenerator has a vessel, a combustor, that burns nearly all the coke to CO2 with little or no CO promoter and with low excess oxygen. The vessel has a highly backmixed fast fluidized bed. A portion of the hot regenerated catalyst from the upper regenerator is recirculated to the lower combustor to heat the incoming spent catalyst and to control the combustor density and temperature for optimum coke combustion rate. As the catalyst and flue gas mixture enters the combustor riser, the velocity is further increased and the two-phase mixture exits through symmetrical downturned disengager arms into an upper chamber of the vessel. The upper chamber separates the catalyst from the flue gas with the disengager arms followed by cyclones and return it to the catalyst bed which supplies hot regenerated catalyst to both the riser reactor and combustor below. The regenerated catalyst recycling provides heat to accelerate the combustion of the lower phase of catalyst. Combustors are advantageous because of their efficient oxygen requirements.
A bubbling bed regenerator carries out the coke combustion in a dense fluidized bed of catalyst. Fluidizing combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Only catalyst entrained in the gas exits the reactor with the vapor. Cyclones above the dense bed separate the catalyst entrained in the gas and return it to the catalyst bed. The superficial velocity of the fluidizing combustion air is typically less than 1.2 m/s (4 ft/s) and the density of the dense bed is typically greater than 480 kg/m3 (30 lb/ft3) depending on the characteristics of the catalyst. The mixture of catalyst and vapor is heterogeneous with pervasive vapor bypassing of catalyst. The temperature will increase in a typical bubbling bed regenerator by about 17° C. (about 30° F.) or more from the dense bed to the cyclone outlet due to combustion of CO in the dilute phase. The flue gas leaving the bed may have about 2 mol-% CO. This CO may require about 1 mol-% oxygen for combustion. Assuming the flue gas has 2 mol-% excess oxygen, there will likely be 3 mol-% oxygen at the surface of the bed and higher amounts below the surface. Excess oxygen is not desirable for low NOX operation.
Refiners often use CO promoter (equivalent to 0.5 to 3 ppm Pt inventory) to control afterburn at the low excess O2 required to control NOX at low levels. While low excess O2 reduces NOX, the simultaneous use of Pt CO promoter often needed for afterburn control can more than offset the advantage of low excess O2.
The superficial velocity of the combustion gas is typically less than 1.2 m/s (4.2 ft/s) and the density of the dense bed is typically greater than 640 kg/m3 (40 lb/ft3) depending on the characteristics of the catalyst. The mixture of catalyst and combustion gas is heterogeneous with pervasive gas bypassing of catalyst.
The dilute transport flow regime is typically used in FCC riser reactors. In transport flow, the difference in the velocity of the gas and the catalyst is relatively low with little catalyst back mixing or hold up. The catalyst in the reaction zone maintains flow at a low density and very dilute phase conditions. The superficial gas velocity in transport flow is typically greater than 2.1 m/s (7.0 ft/s), and the density of the catalyst is typically no more than 48 kg/m3 (3 lb/ft3). The density in a transport zone in a regenerator may approach 80 kg/m3 (5 lb/ft3). In transport mode, the catalyst-combustion gas mixture is homogeneous without gas voids or bubbles forming in the catalyst phase.
Intermediate of dense, bubbling beds and dilute transport flow regimes are turbulent beds and fast fluidized regimes. In a turbulent bed, the mixture of catalyst and combustion gas is not homogeneous. The turbulent bed is a dense catalyst bed with elongated voids of combustion gas forming within the catalyst phase and a less discernible surface. Entrained catalyst leaves the bed with the combustion gas, and the catalyst density is not quite proportional to its elevation within the reactor. The superficial combustion gas velocity is between about 1.1 and about 2.1 m/s (3.5 and 7 ft/s), and the density is typically between about 320 and about 640 kg/m3 (20 and 40 lb/ft3) in a turbulent bed.
Fast fluidization defines a condition of fluidized solid particles lying between the turbulent bed of particles and complete particle transport mode. A fast fluidized condition is characterized by a fluidizing gas velocity higher than that of a dense phase turbulent bed, resulting in a lower catalyst density and vigorous solid/gas contacting. In a fast fluidized zone, there is a net transport of catalyst caused by the upward flow of fluidizing gas. The catalyst density in the fast fluidized condition is much more sensitive to particle loading than in the complete particle transport mode. From the fast fluidized mode, further increases in fluidized gas velocity will raise the rate of upward particle transport, and will sharply reduce the average catalyst density until, at sufficient gas velocity, the particles are moving principally in the complete catalyst transport mode. Thus, there is a continuum in the progression from a fluidized particle bed through fast fluidization and to the pure transport mode. The superficial combustion gas velocity for a fast fluidized flow regime is typically between about 1.5 and about 3.1 m/s (5 and 10 ft/s) and the density is typically between about 48 and about 320 kg/m3 (3 and 20 lb/ft3).
Conversion of gases to reduce NOX requires suitable temperatures, oxygen levels, and mean residence time. Cyclone inlet horns are usually positioned in the same direction, leading to the formation of streamlines of gas, and some catalyst, in the direction of entrance into the inlet horns. Cyclones are typically positioned at the top of both the bubbling bed regenerator and the upper chamber of a combustor-style regenerator. Streamlines generally follow along the wall of the regenerator. In the upper chamber of a combustor-style regenerator, streamlines start from the disengager and curve outward to the wall then follow the wall around and upwardly to enter through the cyclone inlet horns. The center is relatively stagnant and little overall mixing occurs. Mean residence time for the gases is decreased as a result of the streamlines rather than overall mixing. Temperature and oxygen level concentration differences may result.